Low resistivity silicon carbide

ABSTRACT

An opaque, low resistivity silicon carbide and a method of making the opaque, low resistivity silicon carbide. The opaque, low resistivity silicon carbide is doped with a sufficient amount of nitrogen to provide the desired properties of the silicon carbide. The opaque, low resistivity silicon carbide is a free-standing bulk material that may be machined to form furniture used for holding semi-conductor wafers during processing of the wafers. The opaque, low resistivity silicon carbide is opaque at wavelengths of light where semi-conductor wafers are processed. Such opaqueness provides for improved semi-conductor wafer manufacturing. Edge rings fashioned from the opaque, low resistivity silicon carbide can be employed in RTP chambers.

[0001] The present application is a continuation-in-part application ofU.S. patent application Ser. No. 09/790,442 filed Feb. 21, 2001 which isa non-provisional application of Provisional Application Serial No.60/184,766 filed Feb. 24, 2000.

BACKGROUND OF THE INVENTION

[0002] The present invention is directed to an improved low resistivitysilicon carbide. More specifically, the present invention is directed toan improved low resistivity silicon carbide that has a high nitrogenconcentration.

[0003] Silicon carbide (SiC), especially silicon carbide produced bychemical vapor deposition (CVD), has unique properties that make it amaterial of choice in many high temperature applications. Chemical vapordeposition processes for producing free-standing silicon carbidearticles involve a reaction of vaporized or gaseous chemical precursorsin the vicinity of a substrate to result in silicon carbide depositingon the substrate. The deposition reaction is continued until the depositreaches the desired thickness. To be free standing, the silicon carbideis deposited to a thickness of upward of 0.1 mm. The deposit is thenseparated from the substrate as a free-standing article that may or maynot be further processed by shaping, machining, or polishing and thelike to provide a final silicon carbide article.

[0004] In a chemical vapor deposition silicon carbide production run, asilicon carbide precursor gas, such as a mixture ofmethyltrichlorosilane (MTS), hydrogen and argon, is fed to a depositionchamber where the mixture is heated to a temperature at which themixture reacts to produce silicon carbide. Hydrogen scavenges chlorinethat is released from the MTS when the MTS dissociates during thereaction. An inert, non-reactive gas such as argon or helium is employedas a carrier gas for MTS (a liquid at room temperature). Inert gasesalso act as a diluent whose flow rate can be varied to optimize thereaction and assures removal of by products from the reaction/depositionzone. The silicon carbide deposits as a layer or shell on a solidmandrel provided in the deposition chamber. After the desired thicknessof silicon carbide is deposited on the mandrel, the coated mandrel isremoved from the deposition chamber and the deposit separated therefrom.The monolithic free-standing article may then be machined to a desiredshape. Several CVD-SiC deposition systems are described in U.S. Pat.Nos. 5,071,596; 5,354,580; and 5,374,412.

[0005] Pure CVD-SiC has relatively high electrical resistivity. Whilethis is a desirable characteristic for certain applications, such acharacteristic is a limitation restricting its use in otherapplications. Certain components, such as plasma screens, focus ringsused in plasma etching chambers need to be electrically conductive, andedge rings and susceptors used in the RTP systems need to be opaque andpossess high temperature stability. While high temperature properties ofCVD-SiC have made it a material of choice for use in such chambers, itshigh resistivity has limited its use in fabricating components thatrequire a greater degree of electrical conductivity.

[0006] High electrical resistivity of CVD-SiC has further restricted itsuse in applications that are subject to the buildup of staticelectricity. The need to ground components used in such applicationsrequires that they possess greater electrical conductivity than isgenerally found in CVD-SiC. A low resistivity silicon carbide canprovide a unique and useful combination of high temperature propertieswith suitable electrical conductivity properties for use in applicationswhere grounding is required.

[0007] The semiconductor industry uses CVD-SiC components of differentelectrical resistivity. Different resistivity components providedifferent electrical coupling to plasma (i.e. absorbs different amountsof energy from the plasma) in plasma etch chambers. High resistivity SiCcomponents, i.e., 1000 to 5000 ohm-cm, such as gas diffuser plates, donot absorb much energy from the plasma and thus are used in those areasof the etch chamber where plasma energy is not affected. In contrast,low resistivity components such as plasma screens and liners absorbenergy from the plasma and thus prevent plasma from spreading beyond thecomponents. If the plasma is permitted to spread beyond the lowresistivity components, it will generate heat in the system which willdegrade the equipment.

[0008] Regardless of whether the CVD-SiC is of a high or low electricalresistivity, a uniform resistivity is always desired. A uniformelectrical resistivity provides for less variation in the performance ofthe SiC component whether it is functioning as a gas diffuser plate oras an edge ring. A SiC article with uniform resistivity also heats thecomponents uniformly, thus reducing the temperature gradients and thethermal stresses in the material. A uniform electrical resistivity of aSiC component is less susceptible to cracking in extreme environmentsemployed in manufacturing semiconductors such as in the extremeenvironments of rapid thermal processing (RTP).

[0009] U.S. patent application Ser. No. 09/790,442, filed Feb. 21, 2001,(non-provisional of provisionally filed U.S. application No. 60/184,766,filed Feb. 24, 2000), assigned to the assignee of the presentapplication discloses a chemical vapor deposited low resistivity siliconcarbide (CVD-LRSiC) and method of making the same. Electricalresistivity of the silicon carbide is 0.9 ohm-cm or less. In contrast,the electrical resistivity of relatively pure silicon carbide, prior tothe CVD-LRSiC of the application No. 09/790,442, is in excess of 1000ohm-cm. The method of preparing the CVD-LRSiC employs many of the samecomponents as the CVD methods disclosed above except that nitrogen isalso employed. The lower resistivity of the silicon carbide is believedto be attributable to a controlled amount of nitrogen throughout thesilicon carbide as it is deposited by CVD. Controlled means that thenitrogen is maintained at a constant concentration during SiCdeposition. The nitrogen is incorporated in the deposit by providing acontrolled amount of nitrogen with the precursor gas in the gaseousmixture that is fed to the reaction zone adjacent a substrate. Thereaction is carried out in an argon gas atmosphere. Nitrogenconcentration in the gaseous reaction mixture does not exceed 32% byweight of the mixture. At such nitrogen concentrations, the physicalproperties of SiC, other than the electrical resistivity, do not changesuch as to affect performance of SiC articles in thermal processingapplications. As the silicon carbide precursor reacts to form thesilicon carbide deposit, nitrogen from the gaseous mixture isincorporated into the deposit. The CVD-LRSiC contains at least 6.3×10¹⁸atoms of nitrogen per cubic centimeter of CVD-LRSiC. The electricalresistivity of the SiC was below 0.9 ohm-cm. Such low electricalresistivity is highly desirable and are suitable for use in thermalprocessing methods. However, the electrical resistivity of the SiC wasnot as uniform as the industry preferably desires. SiC samples rangeddown to 0.1 ohm-cm with a mean electrical resistivity of around 0.52ohm-cm with a variation around the mean of about 80%. Although such SiCis suitable for thermal processing methods, there is still a need for aSiC with a more uniform electrical resistivity and a lower variationaround the mean.

[0010] U.S. patent application Ser. No. 10/000,975 filed Oct. 24, 2001also assigned to the assignee of the present application disclosesanother chemical vapor deposited low resistivity silicon carbidearticle. The silicon carbide has an improved electrical resistivity ofless than 0.10 ohm-cm and is opaque at wavelengths of from 0.1 μm to 1.0μm at a temperature of at least 250° C. The improvements in theelectrical resistivity as well as the opacity were achieved withoutcompromising other SiC properties such as thermal conductivity, flexuralstrength and thermal stability. The improved electrical resistivity andother properties were achieved by increasing the nitrogen incorporatedinto the SiC. Nitrogen content reached 3×10¹⁹ atoms of nitrogen percubic centimeter of SiC. Nitrogen content was increased by increasingnitrogen volume to as high as 50% in the deposition chamber.Surprisingly, the increase in nitrogen concentration did not compromisethe thermal conductivity or other important SiC properties. Alteringreaction components and chamber conditions may affect the stoichiometryof SiC reactants such that the resulting SiC may have reducedproperties, such as thermal conductivity, and may be prone to cracking.Thus, a worker may not readily predict the quality of SiC that he mayobtain by altering reaction parameters based on previous SiC synthesisprocesses.

[0011] While the resistivity of CVD-SiC can theoretically be lowered toa desired level by the introduction of a sufficient amount ofimpurities, such as boron, the resulting elevated levels of impuritiesadversely affect other properties of the SiC such as thermalconductivity and/or high temperature stability. The CVD-LRSiC isrelatively free of impurities, containing less than 10 ppmw of impuritytrace elements as determined by gas discharge mass spectroscopy. TheCVD-LRSiC is further characterized by a thermal conductivity of at least195 Watts/meter degree Kelvin (W/mK) and a flexural strength of at least390 MPa.

[0012] The electrically conductive CVD-LRSiC possesses high temperaturestability in addition to being a high purity SiC. Thus, the freestanding CVD-LRSiC may be readily employed in high temperature furnacessuch as semiconductor processing furnaces and plasma etching apparatus.The CVD-LRSiC may be sold as a bulk material or may be further processedby shaping, machining, polishing and the like to provide a more finishedfree-standing article. For example, the CVD-LRSiC may be machined intoplasma screens, focus rings and susceptors or edge rings forsemi-conductor wafer processing and other types of high temperatureprocessing chamber furniture as well as other articles where CVD-LRSiCmaterial is highly desirable.

[0013] In the manufacture of semi-conductor wafers, there are numerousprocess steps. One set of steps is referred to as epitaxial deposition,and generally consists of depositing a thin layer (from 10 to less thanone micron) of epitaxial silicon upon the wafer. This is achieved usingspecialized equipment such as SiC wafer boats or SiC susceptors or edgerings to secure the semi-conductor wafers in processing chambers, and achemical vapor deposition (CVD) process. The CVD process requires thatthe wafer be heated to very high temperatures, on the order of 1200° C.(2000° F.).

[0014] There has been a recent trend in the semi-conductor art to employequipment that operates upon a single wafer, rather than a group ofwafers. In single wafer equipment, the heating of the wafer to the CVDtemperature is greatly accelerated such that the wafer is taken fromabout room temperature to an elevated temperature within 30 seconds.Such processing is known as rapid thermal processing or RTP. Rapidthermal processing (RTP), for example, is used for several differentfabrication processes, including rapid thermal annealing (RTA), rapidthermal cleaning (RTC), rapid thermal chemical vapor deposition (RTCVD),rapid thermal oxidation (RTO), and rapid thermal nitridation (RTN).Temperatures in an RTP chamber may exceed 1100° C. and are subject torapid change, thereby making precise control of the substratetemperature complicated and difficult. RTP includes depositing variousthin films of different materials by an RTP-CVD process, rapid annealingof wafers (RTP thermal processing) and rapid oxidation to form silicondioxide. While the silicon wafer can accept such rapid temperaturechange well, the wafer must be held in position by a susceptor or edgering that can also withstand such rapid temperature changes. Susceptoror edge rings composed of CVD-SiC or CVD-LRSiC have proved very suitablefor withstanding RTP conditions.

[0015] Many RTP systems employ high intensity tungsten (W) halogen lampsto heat semi-conductor wafers. Pyrometers are used to measure and tocontrol wafer temperature by controlling the output of the W-halogenlamps. Accurate and repeatable temperature measurements for wafers overa wide range of values are imperative to provide quality wafers thatmeet the requirements for integrated circuit manufacturing. Accuratetemperature measurement requires accurate radiometric measurements ofwafer radiation. Background radiation from W-halogen lamps (filamenttemperature of 2500° C.) or from other sources can contribute to anerroneous temperature measurement by the pyrometer especially at lowtemperatures (400° C.) where the radiant emission from the wafer is verylow compared to the lamp output. Also, any light from the W-halogenlamps that passes through (transmits) a susceptor or edge ring can causean incorrect temperature reading by the pyrometer.

[0016] The industry has addressed the temperature problems by designingthe RTP chamber with single sided heating and mounting the pyrometers onthe chamber bottom opposite the light source. To further reduce lightinterference, the area under the wafer was made “light tight”, thuseliminating stray reflected light from entering the area under thewafer. In addition to redesigning the RTP chamber, CVD-SiC or CVD-LRSiCedge rings and susceptors were made more opaque to W-halogen lamp lightin the wavelength range that pyrometers operate by coating the ringswith about 200 μm (0.008 inches) of poly-silicon to reduce the lamplight that passes through the edge rings. Examples of such edge ringswhich are made more opaque to lamp light are disclosed in U.S. Pat. No.6,048,403 and U.S. Pat. No. 6,200,388 B1. However, although opaquenessmay be increased, coating edge rings with poly-silicon adds substantialcost to the edge rings. Further, the coating process (epitaxial silicongrowth) has many technical problems associated with it such as dendriticgrowth, bread loafing around edges and purity problems that reduceyields. Poly-silicon coating adds thermal mass to the edge rings. Theincreased thermal mass limits heating ramp rates during RTP processingcycles. Ideally, edge rings have a thermal mass that is as low aspossible to achieve the fastest heating ramp rates. The faster the ramprate the shorter the processing cycle time for wafers, thus reducingwafer processing costs. Another advantage to faster ramp rates is thatthe total integrated time at high temperature for the wafers is reducedallowing for less diffusion of any dopant species employed duringprocessing. Such is highly desirable as the feature sizes decrease forsemi-conductor devices (trend in the semi-conductor industry).

[0017] Although there are CVD-LRSiC articles that may be employed insemi-conductor wafer processing chambers, there is still a need forimproved CVD-LRSiC articles that have improved opaqueness and that donot need coatings to achieve a desired opaqueness. Additionally, thereis also a need for CVD-LRSiC articles that have more uniform electricalresistivity.

SUMMARY OF THE INVENTION

[0018] The present invention is directed to a silicon carbide that has anitrogen content greater than 3×10¹⁹ atoms/cm³. Advantageously, thesilicon carbide of the present invention has a resistivity of less than0.10 ohm-cm with a variation around an electrical resistivity mean ofless than 35%. Such a uniform low resistivity silicon carbide providesfor improved performance as furniture in thermal processing methods, andis less susceptible to cracking in extreme environments. Advantageously,the silicon carbide is opaque to light in a wavelength range of from 0.1μm to 1.0 μm. Silicon carbide of the present invention may be employedas furniture in semi-conductor processing chambers where accuratemaintenance of semi-conductor wafer temperatures are desired. Since thelow resistivity silicon carbide is opaque at wavelengths of light from0.1 μm to 1.0 μm, light from heating lamps employed in wafer processingchambers is significantly reduced, thus allowing a more accurate readingof wafer temperatures. Accordingly, defects in wafers caused by impropertemperatures are reduced or eliminated.

[0019] Further, because the low resistivity silicon carbide need not becoated with poly-silicon to provide the appropriate opaqueness, suchproblems as dendritic growth, bread loafing and the high cost of coatingare eliminated. Also, the problem of adding thermal mass to siliconcarbide articles is eliminated. Thus, faster heating ramp rates forrapid thermal processing are achieved with articles prepared from thelow resistivity silicon carbide of the present invention. The fasterheating ramp rates in turn provide for reduced total integrated time athigh temperatures for wafers and reduced diffusion of dopant duringprocessing.

[0020] The present invention is also directed to a method of making theopaque, low resistivity silicon carbide. The silicon carbide of thepresent invention is prepared by chemical vapor deposition. Gas phasenitrogen concentrations of at least 56% by volume are employed in theCVD process. Reactants are mixed together with the high concentrationsof nitrogen in a CVD chamber, and the silicon carbide product isdeposited on a substrate such as a mandrel. Total furnace pressureduring the CVD process is at least 300 torr. Surprisingly, by increasingtotal furnace pressure to at least 300 torr to increase nitrogenconcentration, desirable properties such as thermal conductivity,thermal stability and the like are not compromised. Accordingly, thesilicon carbide articles may readily be employed CVD and RTP processes.

[0021] In addition to having low resistivity and to being opaque tolight at wavelengths of from 0.1 μm to 1.0 μm, the opaque, lowresistivity silicon carbide is stable at high temperatures, has a highthermal conductivity and is of a high purity. The opaque, lowresistivity silicon carbide may be removed from the mandrel and retainedin bulk form or may be shaped, machined and polished to provide a finalarticle.

[0022] A primary objective of the present invention is to provide for afree standing, low resistivity silicon carbide with a nitrogen contentgreater than of 3×10¹⁹ atoms/cm³.

[0023] Another objective of the present invention is to provide for afree standing, low resistivity silicon carbide that is opaque to lightat wavelengths of from 0.1 μm to 1.0 μm.

[0024] A further objective of the present invention is to provide athermally stable free standing, low resistivity silicon carbide.

[0025] An additional objective of the present invention is to provide afree standing, low resistivity silicon carbide that has a high purity.

[0026] Still yet, a further objective of the present invention is toprovide a method of making an opaque, low resistivity silicon carbide.

[0027] Other objectives and advantages of the present invention may beascertained by a person of skill in the art after reading the followingdisclosure and the appended claims.

BRIEF DESCRIPTION OF THE DRAWINGS

[0028]FIG. 1 is a schematic of a CVD apparatus that may be used tofabricate opaque, low resistivity silicon carbide;

[0029]FIG. 2A is a top view of an edge ring composed of CVD depositedopaque, low resistivity silicon carbide; and

[0030]FIG. 2B is a cross-section of an edge ring composed of CVDdeposited opaque, low resistivity silicon carbide.

DETAILED DESCRIPTION OF THE INVENTION

[0031] Chemical vapor deposited (CVD) silicon carbide (SiC) of thepresent invention has a nitrogen content of greater than 3×10¹⁹atoms/cm³. The silicon carbide has low electrical resisitivity of lessthan 0.10 ohm-cm, and is opaque to light at wavelengths of from 0.1 μmto 1.0 μm. Such silicon carbide is bulk or free-standing, i.e.,self-supporting. Bulk or free-standing silicon carbide is distinguishedfrom thin film silicon carbide which is deposited upon a surface withthe intent that the silicon carbide remain permanently bonded to thesurface. Bulk or free-standing silicon carbide can be machined andpolished to a desired size and shape. Such silicon carbide may beemployed as furniture in processing chambers for chemical vapordeposition (CVD), rapid thermal processing (RTP), epitaxial depositionand the like. In addition to having a low resistivity and being opaqueto light at wavelengths of from 0.1 μm to 1.0 μm, the silicon carbidehas a high thermal conductivity, high flexural strength, and highthermal stability. The opaque, low resistivity silicon carbide issubstantially free of metallic impurities containing less than about 3ppmw of trace elements such as boron, phosphorous and the like asdetermined by gas discharge mass spectroscopy. Additionally, the opaque,low resistivity silicon carbide has reduced thermal mass.

[0032] The opaque, low resistivity silicon carbide is prepared bychemical vapor deposition. To provide a substantially opaque, lowresistivity silicon carbide, high concentrations of nitrogen areincorporated into the CVD silicon carbide. High concentrations ofnitrogen are incorporated into the silicon carbide by providing a highconcentration nitrogen atmosphere for the silicon carbide precursors toreact. Nitrogen atmosphere means that no inert, non-reactive gases suchas argon, helium or other noble gas are employed in preparing theopaque, low resistivity silicon carbide. The atmosphere composes atleast 56% by volume of nitrogen, preferably from 56% to 90% by volume ofnitrogen. More preferably the atmosphere composes from 60% to 90% byvolume of nitrogen to obtain the desired nitrogen content and electricalresistivity of silicon carbide. The remainder of the atmosphere iscomposed of hydrogen gas and silicon carbide precursors. Nitrogen isincorporated into silicon carbide in amounts of greater than 3×10¹⁹atoms of nitrogen per cubic centimeter of silicon carbide. Preferably,nitrogen is incorporated into the silicon carbide in amounts of from5×10¹⁹ to 1×10²⁰ atoms/cm³, more preferably from 7×10¹⁹ to 1×10²⁰atoms/cm³. While not being bound to theory, nitrogen is believed to actas a dopant that reduces band gaps in the silicon carbide to reduceelectrical resistivity. Surprisingly, raising the percentage orconcentration of nitrogen to 56% by volume or greater increases theamount of nitrogen incorporated into the silicon carbide withoutnitrogen saturation. Further, the amount of nitrogen incorporation from3×10¹⁹ atoms/cm³ to 1×10²⁰ atoms/cm³ unexpectedly provides for bothdecreased electrical resistivity and a more uniform electricalresistivity. Nitrogen may be employed in any suitable form such asN_(2(g)), volatile organic compounds containing —NO₂ or amine groupssuch as —NH₂, compounds of —N(H)₄ ⁺ and quaternary amines, NO₃ ⁻ saltsin aqueous form, halogen containing nitrogen compounds, and the like.Examples of suitable nitrogen compounds include NF₃ and NH₃.

[0033] Silicon carbide precursor is selected from any suitable materialthat can be reacted to form silicon carbide. Such materials include acomponent, such as a silane or a chlorosilane, which can react to form asilicon moiety and a component such as a hydrocarbon that can react toform a carbon moiety. The component contributing the silicon moiety canbe different from, or can be the same as, the component contributing thecarbon moiety. Hydrocarbon substituted silanes are preferred siliconcarbide precursors because they contain both the silicon carbidemoieties in a single compound. The precursor(s) or reactant(s) can be acompound which dissociates at the reaction conditions forming one orboth of the silicon carbide moieties, or the precursors can be two ormore compounds which react to provide one or both of the moieties. Whilethe precursor(s) needs to be in the gas phase when reacted in thevicinity of the substrate, it is not necessary that the precursor'sboiling point be less than ambient temperature. Methyltrichlorosilane(MTS) is a preferred precursor, especially when used with hydrogen (H₂),which scavenges chlorine released when MTS dissociates. Since MTSprovides both silicon and carbon in a stoichiometric ratio of 1:1, noother source of silicon and carbon moieties is required. Silicon carbideof the present invention may be prepared by any suitable method in theart provided that a nitrogen atmosphere in the reaction chamber is atleast 56% by volume. H₂/MTS molar ratios may range from 2 to 10,preferably from 4 to 10. Hydrogen partial pressures may range from 92torr to 130 torr, preferably 100 torr to 115 torr. MTS partial pressuresmay range from 16 torr to 30 torr, preferably from 20 torr to 25 torr.

[0034] Nitrogen may be provided in any suitable form as long as the formis sufficiently volatile or may be made sufficiently volatile to form agas. Nitrogen partial pressures may range from 191 torr to 675 torr,preferably from 200 torr to 560 torr. No inert, non-reactive gases suchas argon, helium or other noble gas are employed in preparing theopaque, low resisitivity silicon carbide.

[0035] Total deposition chamber or furnace pressures range from 300 torrto 835 torr, preferably from 320 torr to 700 torr. The furnace pressuresequal the sum of the partial pressures of the reactants. Such high totalfurnace pressures provide for the desired nitrogen concentrations in thesilicon carbide. Deposition chamber temperatures range from 1250° C. to1400° C., preferably from 1300° C. to 1375° C.

[0036] Flow rates may vary depending upon the specific apparatusemployed. Generally, the flow rate of MTS ranges from 15 slpm to 50slpm. The flow rate of hydrogen ranges from 25 slpm to 150 slpm, and theflow rate of nitrogen ranges from 70 slpm to 700 slpm.

[0037] Deposition substrates may be composed of any suitable materialthat can withstand the harsh conditions of the deposition chambers. Anexample of a suitable substrate or mandrel for depositing siliconcarbide is graphite. Graphite mandrels as well as other types ofmandrels may be coated with a release agent such that deposited siliconcarbide may be readily removed from the mandrel after deposition.Silicon carbide deposits may be removed from mandrels by controlledoxidation (controlled combustion).

[0038] An example of a chemical vapor deposition system for producingopaque, low resistivity silicon carbide articles of the presentinvention is illustrated in FIG. 1. Deposition is carried out withinfurnace 10. A stainless steel wall provides a cylindrical depositionchamber 12. Heating is provided by a graphite heating element 14 whichis connected to an external power supply by an electrode 16. Graphitedeposition mandrels are arranged within a graphite isolation tube 20 andgas is introduced by means of an injector 22 through the upper end ofthe isolation tube so that the reaction gases sweep along mandrels 18.One or more baffle(s) 24 is used to control the aerodynamics of gas flowthrough furnace 10.

[0039] Line 26, which supplies injector 22, is fed by a nitrogencylinder 28, a hydrogen cylinder 30, and a MTS bubbler 32. Nitrogen isfed by lines 34 and 36 both directly to inlet line 26 and throughbubbler 32. The hydrogen cylinder 30 is connected by line 38 to theinlet line 26. Nitrogen flow through lines 32 and 36 and hydrogen flowthrough line 38 are controlled by mass flow controllers 40, 42, and 44.The MTS bubbler 32 is maintained at a constant temperature by a constanttemperature bath 46. A pressure gauge 48 is connected to a feed backloop that controls the gas pressure of bubbler 32.

[0040] Outlet line 50 is connected to a bottom outlet port 51. Pressurewithin the deposition chamber 12 is controlled by a vacuum pump 52 whichpulls gases through the chamber and a furnace pressure control valve 54operably connected to the vacuum pump. Temperature and pressure withindeposition chamber 12 are measured by thermal probe 58 and pressureindicator 56. Exhaust gases are passed through filter 60 to removeparticulate material upstream of the pressure control valve and througha scrubber 62 downstream of the vacuum pump to remove HCl. Otherexamples of such processes and the apparatus employed are described inU.S. Pat. Nos. 5,354,580. 5,374,412 and 5,474,613, the disclosures ofwhich are hereby incorporated herein in their entireties by reference.

[0041] After deposition the bulk or free-standing silicon carbide may besold in bulk form or further processed by shaping, machining, polishingand the like to form a desired article. Further processing involvesnumerous methods that are well known in the art for free-standingsilicon carbide. Such methods often involve diamond polishing andmachining. An example of an article made from the silicon carbide is asusceptor or edge ring employed to hold or secure semi-conductor wafersfor processing in furnaces or other suitable chambers.

[0042] Advantageously, the free-standing low resistivity silicon carbidebecomes opaque at light wavelengths of from 0.1 μm to 1.0 μm, typicallyfrom 0.7 μm to 0.95 μm. Low resistivity silicon carbide may remainopaque to light at wavelengths of from 0.1 μm to 1.0 μm at temperaturesof from 20° C. to temperatures of up to 1475° C., typically from 300° C.to 1300° C. Opaque means that light transmitted through the siliconcarbide has an absorption coefficient that ranges from 259 cm⁻¹ to 1000cm⁻¹, typically from 270 cm⁻¹ to 800 cm⁻¹. Thickness of such opaque, lowresistivity silicon carbide ranges from 0.05 mm to 2.00 mm, typicallyfrom 0.1 mm to 1.0 mm. At such wavelengths, silicon carbide articles donot substantially transmit light and are highly suitable for furniturein semi-conductor processing chambers. Such chambers include, but arenot limited to, RTP processing chambers where pyrometers that operate atwavelengths of 0.7 μm to 0.95 μm monitor semi-conductor wafertemperatures. Such RTP chambers may operate at temperatures of from 300°C. to 1250° C. Opaque low resistivity silicon carbide of the presentinvention remains opaque to light at wavelengths of from 0.1 μm to 1.0μm at such temperatures. Since the opaque, low resistivity siliconcarbide does not substantially transmit light at wavelengths wherepyrometers operate, temperature readings of semi-conductor wafers aremore accurate when processed on furniture composed of the siliconcarbide of the present invention. Thus, fewer defects occur in processedwafers and both manufacturing and cost efficiency are improved.

[0043] Since the low resistivity silicon carbide is opaque to lightwavelengths at which a pyrometer operates, susceptors or edge ringsemployed in RTP need not be coated with poly-silicon. Accordingly,articles composed of silicon carbide within the scope of the presentinvention eliminate the problems of epitaxial silicon growth such asdendritic growth, bread loafing and any impurities that may occur duringthe poly-silicon coating process. Such impurities may lead tocontamination of semi-conductor wafers during RTP. Elimination of thepoly-silicon coating process also reduces over-all cost of makingsilicon carbide articles as well as processing semi-conductor wafers.

[0044] Additionally, elimination of unnecessary coatings on edge ringsreduces the thermal mass of the edge rings and further improves the RTPin semi-conductor wafer manufacturing. Reduced thermal mass permitsfaster ramp rates in RTP, thus reducing semi-conductor processing cycletime, and reducing wafer processing costs. Further, as ramp ratesincrease the total integrated time at high temperatures for wafers isreduced allowing for less diffusion of any dopant species in the RTP.Reduced diffusion of dopant species into semi-conductor wafers in turnpermits features on the wafers to be decreased in size (an industrygoal).

[0045] Low electrical resistivity of the silicon carbide of the presentinvention is also desirable for radio frequency (RF) heated susceptorsto couple the RF field to the susceptor and for components in the plasmaetch chamber to couple the plasma energy to the component. In addition,due to low resistivity the wafer holders can be grounded and do notbuild a static charge on them. Articles made from the low resistivitysilicon carbide include, but are not limited to, edge rings or susceptorrings, wafer boats, epi susceptors, plasma etch components, focusedrings, plasma screens and plasma chamber walls, and the like. Becausethe silicon carbide of the present invention has a low resistivity, thesilicon carbide may be employed as components in electrical devices suchas electrodes and heating elements. In addition to having a lowelectrical resistivity, the electrical resistivity is more uniform.Chemically vapor deposited low electrical resistivity silicon carbideprepared by the method of the present invention may have an electricalresistivity of less than 0.10 ohm-cm. Preferred silicon carbide preparedby the method of the present invention has an electrical resistivity offrom 0.01 ohm-cm to 0.08 ohm-cm. The average value for the electricalresistivity is from 0.02 ohm-cm to 0.06 ohm-cm with an electricalresistivity variance around the mean of less than 35%, typically from25% to 30%.

[0046]FIGS. 2A and 2B show an edge ring machined and polished from asingle piece of opaque, low resistivity silicon carbide within the scopeof the present invention. Edge ring 100 has a circular circumferencecomposed of main ring component 102 with wafer holding flange 104continuous with main ring component 102. Wafer holding flange 104terminates at edge 106 to form void 108. FIG. 2B is a cross-section ofedge ring 100 along line A-A. Main ring component 102 terminates at anouter surface in support flange 110 that is continuous with main ringcomponent 102. Main ring component 102 terminates at an inner surfacewith flange 112 which is continuous with wafer holding flange 104, thussecuring wafer holding flange 104 to main ring component 102.

[0047] Although FIGS. 2A and 2B show an edge ring as a circular article,any suitable shape may be employed. A circular edge ring is illustratedin FIGS. 2A and 2B because many semi-conductor wafers are in a circularshape. Size and thickness of the edge ring may also vary. However, thethinner the edge ring, the more suitable the ring is for processingsemi-conductor wafers. A thin edge ring can heat up faster in waferprocessing furnaces than a relatively thick edge ring, thus reducing theamount of processing time. Thickness of opaque low resistivity siliconcarbide edge rings may range from 0.1 mm to 1.0 mm, preferably from 0.25mm to 0.5 mm.

[0048] The following examples are intended to further illustrate thepresent invention and are not intended to limit the scope of theinvention.

EXAMPLE 1

[0049] An opaque, low resistivity silicon carbide was produced accordingto a method of the present invention in an apparatus similar to thatillustrated in FIG. 1 with the following process conditions: depositiontemperature=1350° C., total furnace pressure=300 torr, hydrogen partialpressure=92 torr, MTS partial pressure=17 torr, and nitrogen partialpressure=191 torr. The nitrogen partial pressure corresponded to 63.6%of nitrogen in the process. One inch equals 2.54 centimeters. Sixresistivity samples were fabricated from different locations in oneplate (24×29-inches). The resistivity values fell in the range of0.057-0.071 ohm-cm. The average electrical resistivity of the sixsamples was 0.064 ohm-cm. The deposition thickness range was0.24-0.564-inches yielding a deposition rate of 2.7-6.3 mil/hour. Thesilicon carbide was characterized for thermal conductivity by laserflash technique, solid phase N₂ concentration by secondary ion massspectroscopy, and purity by gas discharge mass spectroscopy. The resultsare given below: Property Value Thermal conductivity 232-257 W/mK Solidphase N₂ conc. 3-4.6 × 10¹⁹ atoms/cc Chemical impurity 1.14 (ppmw)

[0050] The above property values showed that large amounts of N₂incorporation did not significantly affect the important properties ofsilicon carbide. The thermal conductivity was high and exceeded 200W/mK. The chemical impurities were less than 2 ppmw. Further, electricalresistivity was below 0.1 ohm-cm. Thus, the silicon carbide had reducedelectrical resistivity values.

EXAMPLE 2

[0051] An opaque, low resistivity silicon carbide was produced accordingto a method of the present invention in an apparatus similar to thatillustrated in FIG. 1 with the following process conditions: depositiontemperature=1350° C., total furnace pressure=400 torr, hydrogen partialpressure=100 torr, MTS partial pressure=16 torr, and nitrogen partialpressure=284 torr. The nitrogen partial pressure corresponded to 72% ofnitrogen in the process. Six resistivity samples were fabricated fromdifferent locations in one plate (24×29-inches). The resistivity valuesfell in the range of 0.04-0.043 ohm-cm. The average resistivity of thesix samples was 0.041 ohm-cm. The deposition thickness range for 78 hourdeposition was 0.172-0.355-inches yielding a deposition rate of 2.2-4.5mil/hour. The material was characterized as in Example 1 and the resultsare given below: Property Value Thermal conductivity 227-283 W/mK Solidphase N₂ conc. 4.2 × 10¹⁹ atoms/cc Chemical impurity 1.85 (ppmw)

[0052] The above property values of CVD-SiC showed that large amounts ofN₂ incorporation did not significantly affect the important propertiesof silicon carbide. As in Example 1, thermal conductivity was high andexceeded 200 W/mK. Chemical impurities were below 2.

EXAMPLE 3

[0053] An opaque, low resistivity silicon carbide was produced accordingto a method of the present invention in an apparatus similar to thatillustrated in FIG. 1 with the following process conditions: depositiontemperature=1350° C., total furnace pressure=650 torr, hydrogen partialpressure=110 torr, MTS partial pressure=17 torr, and nitrogen partialpressure=523 torr. The nitrogen partial pressure corresponds to 80% ofnitrogen in the process. Six resistivity samples were fabricated fromdifferent locations in one plate (24×29-inches). The resistivity valuesfell in the range of 0.015-0.028 ohm-cm. The average resistivity was0.021 ohm-cm. The deposition thickness range for 84 hours of depositionwas 0.134-0.302-inches yielding a deposition rate of 1.6-3.6 mil/hour.The material was characterized as in Example 1 and the results are givenbelow: Property Value Thermal conductivity 220-257 W/mK Solid phase N₂conc. 7.6 × 10¹⁹ atoms/cc Chemical impurity 2.07 (ppmw)

[0054] The above property values of CVD-SiC showed that large amounts ofN₂ incorporation did not significantly affect the important propertiesof silicon carbide. As in Examples 1 and 2 above, silicon carbide madeaccording to a method of the present invention had electricalresistivity values of less than 0.1 ohm-cm and maintained high thermalconductivity values as well as low impurities.

EXAMPLE 4

[0055] Four silicon carbide plates were prepared to show that the methodof the present invention provides for opaque silicon carbide articles inthe light wavelength range of from about 0.10 μm to about 1.0 μm. Twoplates which acted as controls had a thickness of 0.0050 inches. Twoplates that were prepared according to the method of the presentinvention had a thickness of 0.0055 inches.

[0056] The two plates that were prepared according to the method of thepresent invention were prepared according to the method described inExample 1 above except that deposition was stopped when the desiredthickness of each plate was achieved.

[0057] The two control plates were prepared by chemical vapor depositionin a 1.5-m production furnace of Morton Advanced Materials, Woburn,Mass., with the following process conditions: depositiontemperature=1350° C., total pressure=200 torr, hydrogen partialpressure=90 torr, MTS partial pressure=17 torr, and nitrogen partialpressure=93 torr. Deposition rate was 1.5 microns/minute.

[0058] After each test sample was prepared, each sample was mounted in aholder which was placed in front of a laser beam of appropriatewavelength. Opaqueness of each sample was determined by measuring eachsamples light transmission at a wavelength of 0.8 microns and 1.06microns. A detector silicon was placed near the backside of each sampleto measure the transmitted signal from the light source. The lightsource for the 0.8 microns wavelength was a titanium/sapphire laser, andthe light source for the 1.06 microns wavelength was anyttrium/neodymium laser. Spot diameter on each sample was 4 mm.Incidence angle was 0° and pulse rate was 10 Hz. Each sample was exposedto 200 pulses of light at the respective wavelengths. The tests weredone at room temperature (20° C.).

[0059] From the transmission values, the absorption coefficient wascalculated for each sample taking the refractive index of SiC as 2.58 at1.06 micron and 2.61 at 0.8 micron. The following formula was used tocalculate the absorption coefficient:

Absorption Coefficient=−(1/t)ln{[((1−R)⁴/4R ⁴ T ²)+(1/R²)]^(0.5)−((1−R)²/2R ² T)}

[0060] Where t is sample thickness, T is fraction of light transmitted,R is fraction of light reflected which is calculated as follows:

R=[(1−n)²/(1+n)²]

[0061] The results of the test and calculations are given in the tablebelow. Absorption Absorption Sample Transmission Coefficient atTransmission Coefficient at Sample Thickness at 1.06 1.06 micron at 0.8microns 0.8 micron Number (inch) microns (%) (cm⁻¹) (%) (cm⁻¹) Control 10.0050 0.279 429 5.13 199 Control 2 0.0050 0.476 387 4.21 215 Opaque 10.0055 0.038 533 1.72 259 Opaque 2 0.0055 0.052 510 1.2 285

[0062] The data showed that the amount of light transmission in thesamples prepared by the method of the present invention (opaque 1 and 2)had reduced light transmissions and higher absorption coefficient atboth wavelengths. In contrast, the controls which were not prepared bydoping silicon carbide with as much nitrogen had higher transmissionsand lower absorption coefficients at both wavelengths. Accordingly, thesilicon carbide of the present invention had improved opacity oversilicon carbide prepared by the lower nitrogen doping process.

What is claimed is:
 1. An article comprising silicon carbide havinggreater than 3×10¹⁹ atoms of nitrogen/cm³.
 2. The article of claim 1,wherein the silicon carbide has from 5×10¹⁹ to 1×10²⁰ nitrogenatoms/cm³.
 3. The article of claim 1, wherein the silicon carbide hasfrom 7×10¹⁹ to 1×10²⁰ nitrogen atoms/cm³.
 4. The article of claim 1,wherein the silicon carbide has an electrical resistivity of less than0.10 ohm-cm.
 5. The article of claim 4, wherein the silicon carbide hasan electrical resistivity of from 0.10 ohm-cm to 0.08 ohm-cm.
 6. Thearticle of claim 4, wherein average electrical resistivity is from 0.02ohm-cm to 0.06 ohm-cm.
 7. The article of claim 4, wherein a variationaround a mean of the electrical resistivity is less than 35%.
 8. Thearticle of claim 1, wherein the silicon carbide is opaque at awavelength of from 0.1 μm to 1.0 μm.
 9. The article of claim 1, whereinthe article is an edge ring, plasma screen or liner.
 10. An articlecomprising silicon carbide with an absorption coefficient from 259 cm⁻¹to 1000 cm⁻¹ at a wavelength of 0.1 μm to 1.0 μm.
 11. A method of makinga silicon carbide article comprising reacting silicon carbide precursorsin a nitrogen atmosphere of at least 56% by volume of reactants anddepositing the silicon carbide on a substrate.
 12. The method of claim11, wherein the nitrogen ranges from 56% to 90% by volume of reactants.13. The method of claim 12, wherein the nitrogen ranges from 60% to 90%by volume of reactants.
 14. The method of claim 11, wherein a totalpressure is at least 300 torr.
 15. The method of claim 14, wherein thetotal pressure is from 300 torr to 835 torr.
 16. The method of claim 15,wherein the total pressure is from 320 torr to 700 torr.
 17. The methodof claim 11, wherein temperatures range from 1250° C. to 1400° C. 18.The method of claim 11, wherein a source of nitrogen comprises N₂(g),volatile amine compounds, or mixtures thereof.
 19. The method of claim18, wherein a source of nitrogen comprises NF₃, NH₃ or mixtures thereof.20. The method of claim 11, further comprising the steps of removing thesilicon carbide from the substrate, and machining and polishing thesilicon carbide into an edge ring.